[PATCH 4/5] vfio: selftests: Add sysfs_kernel_warn_count_get()

[David Matlack <dmatlack@google.com>]

Add a helper to read /sys/kernel/warn_count, which counts the number of
warnings (e.g. WARN(), KASAN, and UBSAN reports) the kernel has
encountered since boot. Return a negative value if the file does not
exist, e.g. on kernels that predate it.

Factor the code to read an integer from a sysfs file out of
sysfs_val_get_int() so that it can be shared by both helpers.

This will be used in a subsequent commit to detect kernel warnings
triggered by tests.

Signed-off-by: David Matlack <dmatlack@google.com>
---
 .../vfio/lib/include/libvfio/sysfs.h          |  7 +++++++
 tools/testing/selftests/vfio/lib/sysfs.c      | 19 +++++++++++++++----
 2 files changed, 22 insertions(+), 4 deletions(-)

diff --git a/tools/testing/selftests/vfio/lib/include/libvfio/sysfs.h b/tools/testing/selftests/vfio/lib/include/libvfio/sysfs.h
index c9ab1ea8f5a9..1b9711d1bf90 100644
--- a/tools/testing/selftests/vfio/lib/include/libvfio/sysfs.h
+++ b/tools/testing/selftests/vfio/lib/include/libvfio/sysfs.h
@@ -9,4 +9,11 @@ char *sysfs_sriov_vf_bdf_get(const char *pf_bdf, int i);
 int sysfs_iommu_group_get(const char *bdf);
 char *sysfs_driver_get(const char *bdf);
 
+/*
+ * Return the number of warnings the kernel has encountered since boot (e.g.
+ * WARN(), KASAN, and UBSAN reports), or a negative value if the kernel does
+ * not support /sys/kernel/warn_count.
+ */
+int sysfs_kernel_warn_count_get(void);
+
 #endif /* SELFTESTS_VFIO_LIB_INCLUDE_LIBVFIO_SYSFS_H */
diff --git a/tools/testing/selftests/vfio/lib/sysfs.c b/tools/testing/selftests/vfio/lib/sysfs.c
index d88e348a1e0d..71470196f092 100644
--- a/tools/testing/selftests/vfio/lib/sysfs.c
+++ b/tools/testing/selftests/vfio/lib/sysfs.c
@@ -33,15 +33,12 @@ static void readlink_base(const char *path, const char *data_fmt, void *out_data
 	VFIO_ASSERT_EQ(ret, 1);
 }
 
-static int sysfs_val_get_int(const char *component, const char *name,
-			     const char *file)
+static int sysfs_path_get_int(const char *path)
 {
-	char path[PATH_MAX];
 	char buf[32];
 	int ret;
 	int fd;
 
-	snprintf_assert(path, PATH_MAX, "/sys/bus/pci/%s/%s/%s", component, name, file);
 	fd = open(path, O_RDONLY);
 	if (fd < 0)
 		return fd;
@@ -56,6 +53,15 @@ static int sysfs_val_get_int(const char *component, const char *name,
 	return ret;
 }
 
+static int sysfs_val_get_int(const char *component, const char *name,
+			     const char *file)
+{
+	char path[PATH_MAX];
+
+	snprintf_assert(path, PATH_MAX, "/sys/bus/pci/%s/%s/%s", component, name, file);
+	return sysfs_path_get_int(path);
+}
+
 static void sysfs_val_set(const char *component, const char *name,
 			  const char *file, const char *val)
 {
@@ -148,3 +154,8 @@ char *sysfs_driver_get(const char *bdf)
 
 	return out_driver;
 }
+
+int sysfs_kernel_warn_count_get(void)
+{
+	return sysfs_path_get_int("/sys/kernel/warn_count");
+}
-- 
2.56.0.317.g454f8302e1-goog